Method of dope-casting film at high shear rate



June 12, 1962 w. F; HUNTER, JR, ETAL 3, 38,

METHOD OF DOPE-CASTING FILM AT HIGH SHEAR RATE Filed July 10, 1958 2Sheets-Sheet 1 Figz I 1y 14w 1 I MWMEHMM J: PR o R AR T Jerome S.Osmalofl INVENTORS METHOD OF DOPE-CASTING FILM AT HIGH SHEAR RATE FiledJ y 10. 195

June 12, 1962 w. F. HUNTER, JR, ETAL 2 Sheets-Sheet 2 M /ZAWzlkerEHunlegJl: Jrome S. Osmalofl 1 N ENTORS TTOBNEYS United StatesPatent Ofiiee 3,h38,209 Patented June 12, 1962 3,038,299 METHQD OFDOPECASTING FILM AT HIGH SHEAR RATE Walker F. Hunter, Jr., and Jerome S.(lsrnaiov, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.'Y., acorporation of New Jersey Filed July 10, 1958, Ser. No. 747,694 3Claims. (Ci. 18-57) This invention relates to the manufacture of film orsheeting from viscous compositions, and particularly to a method for themanufacture of film and sheeting which have a uniform thickness overtheir entire area and whose surfaces are free from optical defects.

This application is a continuation-in-part of our pending applicationSerial No. 485,988, filed February 3, 1955, and now abandoned.

In the manufacture of sheeting or film support, cellulose estersdissolved in suitable solvents are cast out of a slot hopper onto acontinuously moving casting or receiving surface, i.e., a chromiumplated wheel eighteen feet in diameter or an endless belt, where asufficient quantity of the solvents are evaporated from the dope toallow the material to be stripped off the wheel. While it is commonpractice to cast the desired thickness of film at one time, it is alsoknown to multiply cast a thick film by first forming and partiallydrying a thinner film on a casting surface and thereafter applyinganother coating of the same material to said preformed film to build itup to the desired thickness, see US. Patent 2,374,308. Imperfections inthe optical quality in the sheeting or film support are not generallyencountered at the first coating in this multiple casting techniquesince the dope has an opportunity to flow While on the wheel or othercasting surface. When the second coating is applied, however, the dopeattacks the first layer by solvent action, such that any imperfectionsin the second coating at the time of laydown are frozen in the resultingsupport. One of the major problems in this multiple casting technique,and sometimes in the single casting technique, has been the eliminationof lininess (optically visible lines in the direction of the movingcasting surface). A second problem has been the elimination ofcross-lines (lines perpendicular to the direction of the moving castingsurface). A third problem which is inherent in all dope hoppers havingparallel wall extrusion slots is that they are inherently sensitive totemperature gradients in the dope and it is not possible to obtain afilm thickness uniformity which is satisfactory or as good as indicatedby the mechanical tolerance of the slot width without first homogenizingthe dope to a constant temperature.

One object of the present invention is to provide a method for dopecasting which eliminates, or greatly decreases, the appearance oflininess and cross-lines in the cast film or sheeting and greatlydecreases variations in film thickness due to temperature gradientsacross the width of the dope stream as it is extruded from a hopper.

Another object of the present invention is to provide a method ofcasting film or sheeting which comprises feeding a film dope havingnon-Newtonian characteristics and a viscosity greater than 20 poisesfrom a hopper provided with a relatively long discharge slot throughwhich the dope flows toward a casting surface moving past the exit endof said discharge slot, including the step of subjecting said dopeduring its passage through said discharge slot to a shear ratesufiici-ently high to reduce the sensitivity of the apparent viscosityof the dope to variations in temperature and thereby reduce variationsin uniformity of thickness of the extruded stream along the length ofsaid slot because of any temperature variations in the dope along saidslot.

And a further object is to provide a method of casting film or sheetingas set forth wherein the dope discharged from the hopper slot isaccumulated in an elongated puddle extending along the length of saidslot and from which puddle a layer of dope of uniform thickness is drawnby the casting surface moving at a linear velocity which is a fractionof the velocity of the dope passing through the discharge slot, a highshear rate in the slot being developed by this high velocity.

The novel features that we consider characteristic of our invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its mode of operation,together with additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in which:

FIG. 1 is a diagrammatic side elevation, partly in section and on areduced scale, of a typical film making apparatus by which our inventionmaybe carried out, the hopper being exaggerated in relativesize for thesake of clearness;

FIG. 2 is an enlarged fragmentary sectional view of the mouth of ahopper constructedin accordance with the prior art in combination with afilm casting surface;

FIG. 3 is an enlarged fragmentary sectional view of a hopper constructedin accordance with the present invention in association with a filmcasting surface, and showing the elongated puddle of dope from which thedesired layer is drawn by, the casing surface and which decreases thedistance to laydown of the dope stream; and

FIG. 4 is an enlarged fragmentary sectional view of the discharge end ofa hopper constructed in accordance with the present invention andemphasizing the offset of the hopper lips.

Our invention is illustrated as applied to a film or a sheet castingmachine of the rotary drum or wheel type in which a base it, supports abearing 11 for the shaft 12 of the wheel or drum 13. The peripheral face14 of the wheel 13 forms an endless dope casting surface and movesduring rotation in the direction of the arrow 15 from the dope applyingstation 16 to a stripping station 17 and then back to the dope applyingstation 16. As is well known in the art, the dope is applied to thecasting surface 14 in a thin layer of film 18 which dries as the wheelrotates and is later stripped off at the stripping station 17 andremoved over the roller 19.

The dope 20 from which the sheet or film 18 is formed is customarilycontinuously fed into a hopper 21 disposed above the wheel surface andfrom which it flows in the form of a thin stream 22 through a dischargeslot 23" whose length is equal to the width of the wheel surface.According to the present invention, the hopper 21 must be of thepressure type, as shown in FIG. 1, Where the dope is pumped into aclosed chamber 122 under pressure through a pipe 123 and is forcefullyextruded through the discharge slot. In prior art casting apparatus thestream of dope leaving the discharge slot of the hopper is depositeddirectly onto the wheel surface, see FIG. 2, and the width of theopening of the discharge slot more or less controls the thickness of thedeposited film and the finish on the hopper blades is all important tothe smootnness of the surfaces of the deposited film. An inspection ofFIG. 2 will show that with prior art hoppers the velocity of the castingsurface, which is equal to, or greater than, the dope velocity in theslot, acts to pull out the dope stream a considerable distance before itmakes contact with the casting surface. This distance is commonlyreferred to as the distance to laydown and is indicated at X in FIG. 2for prior art hoppers. This unsupported stream of dope has anopportunity to vibrate and cross'lines in the deposited layer result.This tendency is greatly increased in cases where coatings being laiddown are very thin, i.e., 0.002 inch dry, and such cases are prevalentin multiple casting techniques where the final thickness of the film isbuilt up in two or more successive layers. Furthermore, even if thewalls of the extension slot are machined and/or adjusted as accuratelyas mechanically practical, the uniformity of thickness of the dopestream extruded therethrough may be objectionable due to changes inapparent viscosity of the dope resulting from temperature variationsacross the length of the discharge slot. It has been found that a 2 F.variation in dope temperature across the length of the slot produces achange in the apparent viscosity of a dope which will result in anobjectionable variation in thickness of the dope stream.

We have invented a method for casting film or sheeting which eliminatesor greatly reduces the above difficulties found in prior art procedures.Since cellulose ester dopes from which sheeting and film are formed arequite viscous, non-Newtonian materials, then their viscosities are notaffected only by temperature, as in the case of water, but also by shearrate. Therefore, dope viscosities are generally termed apparentviscosities when the dope is in motion and are calculated at varioustemperatures from equations relating to flow rates, density, pressures,etc. The apparent viscosity of such a dope at a particular temperatureand shear rate is not the same as the viscosity at the same temperaturebut another shear rate, or at the same shear rate and a differenttemperature. Generally, viscosity of dope changes rapidly withtemperature, but this condition does not hold over the entire range ofapplication of dopes. It has been shown experimentally that at highshear rates the apparent viscosity of the dope is generally lesssensitive to temperature change than at lower rates. Thus, the volumerate of flow will be more uniform with a high shear rate slot than witha larger opening. The method of casting in accordance with the presentinvention makes use of this discovery by subjecting the dope to a highshear rate at the time of extrusion so as to make its apparent viscosityless sensitive to temperature variations across the length of thedischarge slot of the hopper. A decided improvement in the thicknessuniformity of the dope stream leaving the discharge slot is achieved byvirtue of this technique.

The most practical way of subjecting the dope to a sufiiciently highshear rate in the discharge slot of the hopper is to restrict the widthof the hopper slot as much as possible thus increasing the linearvelocity or shear rate, for a given volume of dope passing through theslot over a given time interval. However, when the hopper discharge slotis reduced to achieve this high shear rate, then the dope stream leavingthe hopper can no longer be extruded directly onto the casting surfacebecause the thickness of the deposited layer would then be equal to, orless than, the width of the discharge slot, and which width wouldnecessarily be less than the wet thickness of dope required to be laiddown in casting film and sheeting. On the other hand, where the width ofthe discharge slot controls the wet thickness of the cast film, it isnot possible to obtain the high shear rates required to make theapparent viscosity of the dope sufficiently insensitive to temperaturevariations along the slot to obtain a uniformly thick dope stream.

This problem is solved according to the present invention by feeding thefilm dope at a high shear rate into an elongated puddle or wave (anaccumulation of dope extending along the entire length of the exit endof the hopper discharge slot and having uniform dimensions and/or volumeat all points along its length) from which the casting surface pulls alayer of film. The thickness of this cast layer can then be a multipleof the width of the discharge slot and the thickness can be readilyvaried by merely changing the linear speed of the casting surface. Allthat is necessary for obtaining a cast film of uniform thickness withthis method is that the dimensions of this elongated puddle be uniformthroughout its length, or the volume of dope in any finite length of thepuddle be the same as the volume in any other finite length of the samevalue at each and every point along the length of the puddle. Thiscondition is achieved by reason of the fact that the discharge slot inour arrangement acts merely as a metering slot to feed dope to thispuddle and because of the dope being subjected to a high shear ratewhenpassing therethrough any inequalities in the volume of dope alongthe length of the slot which might be due to viscosity changes resultingfrom temperature variations along the length of the slot are greatlydecreased.

This method of film casting depends upon the use of a film dope havingsuflicient film strength to puddle as it leaves the discharge slot ofthe hopper. Generally speaking, the film strength and viscosity of filmdopes such as we are concerned with are analogous, so that a dope whichwill satisfy this requirement can be specified by its viscosity. If thismethod of film casting is attempted with solutions having too low aviscosity or film strength then the solutions would not puddle but wouldsplatter or form rivulets upon being extruded through the hopperdischarge slot so that there would be no puddle from which the castingsurface could draw the desired layer of dope. We have found that a filmdope must have a viscosity of 20 poises or greater in order to make thismethod of casting film or sheeting operative.

This method of metering dope to an elongated puddle from which it ispulled by the casting surface also eliminates, or greatly decreases, theappearance of lininess and cross-lines in the cast film or sheeting.Since the discharge slot of the hopper merely meters the dope into thepuddle, any imperfections which might appear in the sheet due toinaccuracies in the edges of the discharge slot will disappear in thepuddle by virtue of the fact that the dope has a chance to flow togetheror merge at this point before it is pulled out onto the casting surface.Furthermore, as explained below, with this method of casting, thedistance to laydown of the layer of dope on the casting surface can becontrolled to some extent so that the stream of dope between the puddleand casting surface is restrained to be relatively short and not be soapt to be subject to vibrations which tend to introduce cross-lines inthe cast layer as mentioned above.

Referring now to the drawings, we will describe a preferred embodimentof an apparatus by means of which a new method of casting can be carriedout. Looking at FIGS. 1, 3 and 4, the discharge slot 23 in the hopper ispreferably relatively narrow compared to the discharge slot of prior arthoppers for casting the same thickness of film from the same dope inorder to subject the stream of dope to a high shear rate (high rate offlow) as it passes therethrough. As shown in FIGS. 3 and 4, thedischarge slot 23 is formed by straight parallel walls 25 and 26' on thefront and rear blades 26 and 27, respectively, of the hopper. The bladeof the hopper to which the casting surface 14 first approaches isdesignated the rear blade 27, while the blade 26 from which the castingsurface last leaves is designated the front blade. According to thepresent invention, the dope must be passed through the discharge slot soas to subject it to a shear rate sufficiently high to reduce thesensitivity of the apparent viscosity of the dope to temperaturevariations along the length of said slot whereby equal volumes of dopewill be supplied along the entire length of the slot despite anytemperature gradient therealong. Shear rate through a slot can bedefined by the equation which is analogous to Newtonian shear rate in apipe as derived from velocity distribution equations, wherein V is theaverage velocity of the dope through the slot and a is the width of theslot opening. In accordance with the present invention, the shear rateto which the dope is subjected to derive this advantage of reducing thesensitivity of the apparent viscosity to temperature changesshould besuch that is equal to at least 70 reciprocal seconds. We have found thatthis shear rate condition can be readily obtained if the width of thedischarge slot 23 of the hopper has a maximum width of 0.015".Substituting the values 0.015 and 70 reciprocal seconds in the formulashear rate=% and solving for V it will be found that V=5.25 ft./-rnin.This :is the lower limit of linear velocity at which the dope must bepassed through the hopper slot to obtain the shear rate necessary withthe specified film dope to reduce the sensitivity of the apparentviscosity of the dope to temperature variations which might occur alongthe length of the slot so that equal volumes of dope will be metered atall points along the slot into the puddle. This size slot issatisfactory when casting films having a thickness of 0.030"-0.038".Obviously, a more narrow discharge slot can be used to advantage whencasting thinner films.

Inasmuch as the dope is extruded from the hopper at a much greaterlinear velocity than it is being taken away by the casting surface, andthe wet thickness of the cast dope is greater than the width of thedischarge slot, some means must be provided for accumulating the dope asit leaves the discharge slot and accumulating it in such a way that auniform layer of dope can be drawn therefrom by the casting surface.Providing the dope has suflicient viscosity or film strength, it couldbe allowed to balloon out of the discharge slot into a self-supportingelongated puddle from which the casting surface could draw a layer ofdope. However, we prefer to provide a physical support for this puddleof dope and, therefore, have shown the hopper lips formed so as toprovide such a physical support for this accumulation of dope and to aidin the formation of such a puddle which has velocity components whichproduce a quicker laydown of the dope stream.

Looking now at FIGS. 3 and 4, the lip 28 of the front blade of thepresent hopper is offset from lip 29 of the rear blade by a distance Z.We have found that for the best results this offset Z should be at least0.030" and not more than 0.070", 0.050 being the offset most often used.Dope leaving the discharge slot 23 puddles (piles up in an orderlyfashion or forms a wave), as shown at 30, on the exposed portion of theback blade 27 and/ or in the air prior to moving onto the castingsurface. The layer of dope is then drawn from this elongated puddle ofdope by the casting surface 14 by virtue of the adhesion of the dope tothe casting surface and the film strength of the dope. In order to starta casting operation, the layer of dope must be drawn to the castingsurface from the puddle by any suitable means to initially form thelayer and fasten it to the casting surface, and from then on a uniformlayer is pulled from this puddle by the casting surface.

If the discharge slot 23 meters the dope uniformly into the puddle 30 soas to maintain the dimensions or volume of the puddle equal at allpoints along its length, then the layer of dope pulled out of the puddleby the casting surface will be uniformly thick. If there are anyimperfections in the stream of dope leaving the discharge slot due toblade finish, they have an opportunity to smooth out in the puddle 30before reaching the casting surface. A further advantage obtained byvirtue of the disclosed construction of the hopper discharge slot 23 isthat a quicker laydown of the dope stream from the exit end of thedischarge slot and the casting surface is obtained than with prior arthopper constructions so that there is less chance for this unsupportedstream of dope to vibrate and cause cross-lines in the finished film orsheet. In use, the velocity component of that portion of the dope at thetop of the puddle 30 is larger than that of the portion of the dopemoving along the exposed portion of the rear blade 27 because of theresistance (friction) of the metal. As a result of this difierence invelocity, at the end of the puddle there is a velocity force tending tothrow the dope stream leaving the puddle down onto the casting surface.This effect tends to counteract the pull-out of the dope stream causedby movement of the casting surface and a decided decrease in thedistance to laydown Y is achieved as compared with the distance tolaydown X characteristic of the prior art hoppers when using the samedope, pressures and casting speed. Shortening this distance to laydowneliminates to a large degree the cross-line pattern inherent in priorart hoppers. This eliminates the necessity of using a vacuum for thesame purpose as disclosed in US. Patent 2,681,294.

In addition, the puddling characteristic of this hopper design allowsvery thin deposits, i.e., 0.005" wet (.001" cured thickness) or less, aswell as heavy deposits, i.e., 0.040" wet (.008" cured thickness) or moreto be made equally well.

The reduction in the sensitivity of the apparent viscosity of the dopedue to temperature variations along the length of the discharge slot isanother advantage derived from the present invention. Many non-Newtonianfluids exhibit this advantage when cast in accordance with the presentinvention. Exceptions are those non-Newtonian fluids having a viscositybelow 20 poises or insuflicient film strength, because such fluids, dueto their low viscosity or lack of film strength, would be delivered fromthe discharge slot of the hopper in a non-usable, uneven layer, or evensplattered, rather than being extruded in a uniform stream in which casethis advantage would be useless even if exhibited. Furthermore, such lowviscosity fluids would not have the puddling characteristic necessary tocarrying out the present invention. Fluids of such low viscosity, below20 poises, do not come under the film dope category anyway. Examples oftypical film dopes which can be, and have been, cast in accordance withthe present invention are as follows:

Example 1 Cellulose acetate (43.5% acetyl) parts 100 Methylene chloridedo 426 Cyclohexane-520 do- 42 Methyl alcohol do 52 Triphenylphosphate do15 Viscosity at F poises 600 Example 2 Cellulose acetate butyrate (17%butyryl) parts Methylene chloride do 280 Cyclohexane350 do 35 Butylalcohol do 35 Triphenylphosphate do 8 Viscosity at 80 F poises 520 Theinvention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be elfected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:

1. The method of casting film or sheeting comprising the steps ofextruding a film dope having non-Newtonian characteristics and aviscosity greater than 20 poises under pressure through a narrowelongated metering slot having a length substantially equal to the widthof the film to be cast at a linear velocity of at least 5.25 ft./min. tosubject the dope to a shear rate sufficiently high to reduce thesensitivity of the apparent viscosity of the dope to temperaturevariations which may exist along the length of said slot whereby thevolume of dope metered by said slot is substantially equal at all pointstherealong regardless of temperature variations, accumulating the dopemetered through said slot into an elongated puddle substantially equalto the length of said slot, and drawing an unsupported layer of dope ofthe desired wet thickness out of said puddle and depositing it onto acasting surface spaced laterally from said puddle by moving said castingsurface at a uniform linear speed transversely relative to said puddle;and stripping said layer of dope from said casting surface when it issufficiently cured.

2. A method of casting film or sheeting according to claim 1 in whichthe film dope is accumulated in such a Way that the puddle is physicallysupported on the side from which the casting surface approaches and theside from which the casting surface leaves is unsupported, and thatportion of the dope moving into and through this unsupported part of thepuddle possesses a higher velocity than the portion of the dope at thesupported side of the puddle which acts to reduce the distance tolaydown of the unsupported layer of dope between the puddle and thecasting surface.

3. A method of casting film or sheeting according to claim 1 in whichthe width of the metering slot is a fraction of the Wet thickness of theunsupported layer of dose deposited on said casting surface and has amaximum width of 0.015 inch.

References Cited in the file of this patent UNITED STATES PATENTS1,921,953 Stephens Aug. 8, 1933 2,364,552 Reichel Dec. 5, 1944 2,659,103Merritt Nov. 17, 1953 2,681,294 Beguin June 15, 1954 2,714,745 KenyonAug. 9, 1955 2,761,791 Russell Sept. 4, 1956

